MAMMALIA, 533 



CHAPTER XXIX. 



6. SLOTH. 7. MOLE. 8. PORPOISE. 9. BAT. 



VI. — The Sloth {Bradypus tridadyhis)—KxhQx^d\, 



The sloths are truly arboreal mammals, being very com- 

 pletely adapted to a tree life. The hair is shaggy and 

 its neutral tint is much in harmony with its surroundings, 

 the more so in those species which cultivate the growth of a 

 green alga upon the hair. The external ears are reduced, 

 probably for easy passage through boughs, and the tail is 

 vestigial. The fore-limbs are longer than the hind, as one 

 tendency of the arboreal habit is the greater use of the fore- 

 limbs. The sloths are herbivorous, feeding solely upon the 

 leaves of trees, and they belong to one of the lowest orders 

 of the Eutherla, namely, the Edentata. We shall therefore 

 expect to find in them certain " edentate" characters, others 

 due to a herbivorous diet, and yet others correlated with the 

 arboreal habit. 



The skull has several peculiarities. The zygomatic arch 

 is not complete, as the jugal does not reach back to the 

 squamosal, and the premaxillae are nearly absent, which 

 assists in reducing the facial part of the skull in proportion 

 to the cranial. The incisor teeth (on premaxilla and oppo- 

 site it) are absent, and so most probably are the canines 

 (though in the two-toed sloth the first tooth is long and 

 pointed Hke a canine). There are five stump-like homodont 

 teeth in the upper jaw and four in the lower jaw. These 

 grow from persistent pulps, as they are w^orn aw^ay and have 

 no enamel. An outer layer of cement envelops the hard 

 thin coat of dentine, which in its turn encloses a softer vaso- 

 dentine. In use the same principle is involved as in other 

 herbivorous types, the hard dentine here playing the part of 

 enamel and forming the slowly-w^earing ridges between cement 

 and vasodentine. So far as is known there is no milk 

 dentition. It is difficult to say hovv far these peculiar dental 

 characters are due to degeneration from a higher Eutherian 

 type, or how far they are due to a primitive condition. 



